Deep Vein Thrombosis, or “DVT”, is a blood clot (“thrombus”) that forms in a vein deep in the body. A thrombus occurs when blood thickens and clumps together. Most of these thrombi occur in the lower leg or thigh; however, they can also occur in other parts of the body. Thrombi located in the thigh are more likely to break off and cause a pulmonary embolism (“PE”) than clots in the lower leg or other parts of the body. The clots that form close to the skin usually cannot break off and cause a PE due to their reduced size and the reduced pressures exerted on them.
A DVT, or a portion of it, can break off and travel through the bloodstream where it can enter the lung and block blood flow. This condition is called pulmonary embolism, which is considered to be very serious due to its likelihood of causing damage to the lungs and other organs and can quite possibly lead to death. This condition affects more than 2.5 million Americans each year and is associated with an estimated 50,000 to 200,000 deaths annually.
The venous system is designed to allow for the return of blood to the right side of the heart. Veins are not passive tubes through which blood passes, but are a system that uses muscular compressions, gravity, and inter-venous valves to promote and control the flow of blood through them. The valves are located along the entire length of the vein and ensure that blood only flows in one (1) direction, toward the heart. Blood flow may easily pass through the valve in the direction toward the heart, but when pressure is greater above the valve than below, the cusps will come together thereby closing the valve and stopping the flow of blood away from the heart.
The valves consist of two (2) very thin-walled cusps that originate at opposite sides of the vein wall and come together to meet at the midline of the vein. The diameter of the vein is slightly larger just behind a valve where the cusps attach to the vein wall. Due to the larger diameter of the vein and the propensity for blood to collect and stagnate between the valve cusps and the vein wall, thrombi formation in this area is more likely.
The most common causes of DVT are venous stasis, blood vessel wall injury, and hypercoagulability. Venous stasis is the reduction of blood flow, most notably in the areas of venous valves, usually caused by extended periods of inactivity. These periods of inactivity minimize the muscular compressions applied to the veins therefore removing the forces used to propel the blood through the veins. This reduction in flow allows the blood to collect and congeal thereby forming a clot. The conditions that contribute to venous stasis include heart disease, obesity, dehydration, pregnancy, a debilitated or bed-ridden state, stroke, and surgery. Stasis has been known to develop with surgical procedures lasting as little as thirty (30) minutes.
Vessel wall injury can disrupt the lining of the vein thereby removing the natural protections against clotting. The loss of natural protection will increase the chances of clot formation and the subsequent mobilization of the clot that can lead to a PE. Some of the major causes of vessel wall injury are trauma from fractures and burns, infection, punctures of the vein, injection of irritant solutions, susceptibility to DVT, and major surgeries.
Hypercoagulability exists when coagulation outpaces fibrinolysis, which is the body's natural mechanism to inhibit clot formation. When this condition exists, the chances of clot formation, especially in areas of low blood flow, are greatly increased. Some causes of hypercoagulability are trauma, surgery, malignancy, and systemic infection. A typical treatment is the administration of an anti-coagulant such as of low-molecular-weight heparin.
It is recognized that clots usually develop first in the calf veins and “grow” in the direction of flow in the vein. The clots usually form behind valve pockets where blood flow is lowest. Once a clot forms, it either enlarges until it is enveloped, which causes the coagulation process to stop, or the clot may develop a “tail” which has a high chance of breaking off and becoming mobile where it can enter the pulmonary system and become lodged in the lungs.
In a patient with DVT, the goals are to minimize the risk of PE, limit further clots, and facilitate the resolution of existing clots. If a potential clot is suspected or detected, bed rest is usually recommended to allow for the clot to stabilize and adhere to the vein wall thereby minimizing the chance of the clot becoming mobile where it can travel to the lungs. A more effective preventative measure is ambulation, which is to walk about or move from place to place. Ambulation requires muscle movement. The muscle movement will provide a continuous series of compressions to the veins thereby facilitating the flow of blood. The continuous flow of blood will reduce or eliminate any areas of stasis so clots do not have a chance to form. For people who are confined to a bed or will be immobile for an extended period of time, leg elevation is recommended. This will promote blood return to the heart and will decrease any existing venous congestion.
Graduated compression stockings have also been used to apply pressure to the veins so as to reduce or minimize any areas of low flow in the vein and not allow the collection and coagulation of blood in these low flow areas. The stockings are designed to provide the highest level of compression to the ankle and calf area, with gradually decreasing pressure continuing up the leg. The stockings prevent DVT by augmenting the velocity of venous return from the legs, thereby reducing venous stasis. Typically, stockings are applied before surgery and are worn until the patient is fully able to move on their own. The stockings need to fit properly and be applied correctly. If too tight, they may exert a tourniquet effect, thereby promoting venous stasis, the very problem they intend to prevent. If too loose, the stocking will not provide adequate compression.
Another treatment of DVT involves the use of intermittent pneumatic compression (IPC). IPC can be of benefit to patients deemed to be at risk of deep vein thrombosis during extended periods of inactivity and is an accepted treatment method for preventing blood clots or complications of venous stasis in persons after physical trauma, orthopedic surgery, neurosurgery, or in disabled persons who are unable to walk or mobilize effectively.
An IPC uses an air pump to inflate and deflate airtight sleeves, or garments, wrapped around the leg. The successive inflation and deflations simulate the series of compressions applied to the veins from muscle contractions thereby limiting any stasis that can lead to thrombi formation. This technique is also used to stop blood clots from developing during surgeries that will last for an extended period of time.
In order to deliver proper and safe medical therapy to the patient, the air pump used in IPC systems must have necessary qualities, characteristics, durability and overall performance capabilities. The pump must reliably create a user-specified pressure in the compression sleeve on the patient, and maintain it within a narrow range for a specified time period with minimal variability, in time or pressure, through countless repetitions of inflation and deflation. To avoid issues of medical concern, such as tissue hypoxia or structural damage, the pump must be able to sense over-inflation of the garment beyond the set pressure, and decrease pressure through slight deflation or by signaling the user to make appropriate changes.
Additionally, the portability, and thus versatility, of an IPC system is important, and is limited by the air pump, typically due to AC power requirements and/or physical size. In care facility, home therapy and hospital settings the patient typically needs to be moved or transferred between rooms or buildings. Such situations can present a significant period of time during which no compression therapy is occurring, creating an increased risk of clotting, DVT and possible resultant PE.
Another version of IPC is the Venous Foot Pump which provides an alternative to the traditional thigh or calf compression device. The foot pump mimics the natural effects of walking and weight-bearing on the circulation in the feet and legs through compressions applied to the foot.
PE remains the most common preventable cause of death in hospitalized patients. The deaths are most often a complication resulting from the formation of a DVT and the subsequent PE that may result from it.
In light of the above, it would be advantageous to provide a deep vein thrombosis prevention system with an air pump that minimizes the occurrence of deep vein thrombosis formation. It would be further advantageous to provide a deep vein thrombosis prevention system having an air pump that allows medical personnel to customize the compression of limbs being treated to optimize treatments for particular patients. It would be further advantageous to provide a deep vein thrombosis prevention system having an air pump that is compact, portable and powered by its own DC battery or standard AC electricity. It would be further advantageous to provide a deep vein thrombosis prevention system having an air pump that is easy to use, relatively easy to manufacture, and comparatively cost efficient.